1. Technical Field
This invention relates to a method and an apparatus for the formation of fiber reinforced polymerized components.
2. Discussion
Fiber reinforced plastic (FRP) parts are being increasingly considered for use in a wide variety of applications. An FRP part generally consists of a plastic shape in which carbon, glass fiber, or other reinforcing fibers are dispersed in order to provide strength to the resin.
FRP products made from thermosetting resins, particularly in sheet molded compound (SMC) form can be formulated to have critical characteristics that are similar to the steel parts they are intended to replace. For example, SMC parts generally have a coefficient of thermal expansion which is equivalent to that of steel and are able to sustain E-coat (metal protection) temperatures of 400 degrees F. Furthermore, SMC parts can be installed by mechanical fasteners or by bonding to metal in the production body shop of an original equipment manufacturer side-by-side with steel body panels.
There are also competing thermoplastic materials which can be used instead of thermosetting resins. A number of thermoplastic materials, both reinforced and non-reinforced, have been evaluated in the automotive industry for primarily vertical components such as fenders and outer door panels. This class of materials has generally been limited to vertical panels because their stiffness is not sufficient (even in the reinforced version) for horizontal panels such as hoods and decklids.
These attempts at using thermoplastics have not been totally successful because they are not compatible with the automotive assembly line processes as have been the thermosets. The thermoplastics generally have to be processed separately from the E-coat bake ovens, and require greater-than-desired gaps between mating surface panels to allow for their high coefficient of thermal expansion.
There are three primary processes to produce higher volume chopped fiber thermoset composite components, and there are features of each process which allow one to be the selected choice for a particular application. These three processes are compression molding, injection molding, and transfer molding.
In compression molding, a charge such as a sheet molded compound containing a curable resin is placed between upper and lower heated die members defining a mold cavity. The dies are then brought to a closed position during which the dies compress the charge causing it to flow and fill the mold cavity. After the resin cures, the dies are opened and the finished part is removed. Compression molding has been historically the process of choice in making fiber reinforced thermoset composite components which require surface finish, mechanical properties, and dimensional stability.
In thermoset injection molding, a plastic is injected into a cavity defined between two die halves. After cross-linking of the polymers is completed, the dies are separated and the finished part is ejected. Injection molding offers design flexibility benefits through enhanced part integration.
In transfer molding, a charge is deposited into a preheating transfer pot. A movable platform is used to drive the heated, flowable charge out of the transfer pot and through a series of channels into a plurality of mold cavities. Transfer molding is used for parts that are too small and intricate for compression molding.
Each approach of the known prior art suffers some disadvantages. Compression molding is relatively expensive. Injection molding and transfer molding cause severe degradation in mechanical properties of the resulting component because of deterioration in the integrity of the chopped fibers resulting from transportation of the material.
Accordingly, an apparatus and process which enables the molding of fiber reinforced plastic components of relatively high degrees of complexity without the strength of the resulting product being compromised because of deterioration of the fibers during the molding process remains wanting.